Working chairs are known, widely used by video terminal operators, typists, etc. Because of the different requirements of different operators and the different utilization conditions, the chair must be able to be adjusted on the basis of these requirements. They must provide each operator with the most comfortable conditions. This is essential because of the lengthy periods for which they are in continuous use.
One of the most frequently required adjustments for working chairs relates to the rearward inclination of the back-rest relative to the seating position.
This adjustment is currently achieved by hinging together a chair structural element rigid with the back-rest and a chair structural element rigid with the seating portion on a vertical axis. The two structural elements have slotted plates, alternating with each other, which are traversed by a single pin and maintained together, able to lock the seating portion and back-rest together in the desired position.
It is also known to interpose between the seating portion and back-rest a spring which, in the absence of other forces, maintains the back-rest in an erect position relative to the seating portion. This position is changeable by a rearward thrust by the operator, followed by locking in the desired position.
A drawback of this known arrangement is that locking the back-rest relative to the seating portion requires a certain force and is uncomfortable for persons lacking strength.
A further drawback is that if the plates are not correctly locked, the back-rest is free to incline relative to the seating portion.
A further drawback is that the locking device requires a large number of parts to be assembled together.
In order to eliminate or at least attenuate these drawbacks, and in particular to facilitate the locking of the palates, it has been proposed to replace the knob associated with the screw means with a spring which maintains the pack plates coupled together and is associated with a lever or eccentric for it temporary deactivation.
This arrangement has proved more advantageous than the preceding but has not eliminated the drawbacks of having a large number of components to be assembled together and the need for the operator to set the back-rest into the desired position by means of his back.
Moreover, in known working chairs it is often requested that in addition to being adjustable relative to the seating portion when in the non-loaded state, the back-rest is also elastically rearwardly yieldable, with the degree of yieldability adjustable on the basis of the weight of the person seated. This is generally achieved by using the spring that returns the backrest into the erect position, when the members for locking the back-rest to the seating portion are deactivated.
A drawback of this arrangement is that using the same spring both for returning the back-rest into the erect position during its adjustment and for counteracting the rearward elastic yielding of the back-rest means that this spring unnecessarily stresses the back-rest inclination adjustment members. These members in themselves do not require any stressing, with the result that they have to be unnecessarily over-dimensioned.
A further drawback is that when the chair is not loaded, i.e., without an operator seated on it, and the member which locks the back-rest relative to the seating portion is voluntarily or accidentally slackened, the return spring, which is sized for a person, suddenly returns the back-rest into the erect position, with possible personal harm.
Another drawback is that if the system for adjusting the rearward inclination of the back-rest is stressed directly by a spring, it becomes difficult to adjust. The adjustment point is determined by the condition of balance between the two opposing forces due, on one hand to the rearward thrust by the operator and, on the other hand, to the elastic reaction of the stressed spring.